Wavelength division multiplexing (WDM) is commonly used in lightwave communications systems to provide increased transmission capacity. As is known to those skilled in the art, the addition of an optical add/drop capability in WDM-based systems provides added flexibility for removing and adding individual channels at intermediate nodes in the WDM transmission path, which further enhances the management of optical transmissions in lightwave communications systems.
In general, most prior art optical add/drop multiplexers (ADMs) utilize fixed or tunable fiber gratings to provide the necessary wavelength selectivity for the add/drop function. These prior art ADMs, whether of the fixed or tunable type, suffer numerous disadvantages, including: path loss for added, dropped, and "through" wavelengths; high implementation costs; and numerous design limitations. Some prior art ADMs attempt to compensate for losses by utilizing optical circulators and fiber gratings in conjunction with a "complete" optical amplifier, commonly referred to as a "lumped" amplifier. This type of ADM typically includes fiber gratings disposed between a first and second optical circulator with a "lumped" amplifier at the input side of the first circulator. The lumped amplifier at the input side is able to provide gain for the optical signals that are dropped via the first circulator as well as those optical signals that pass through the ADM without being dropped. However, the optical signals that are added via the second circulator do not pass through the lumped amplifier. Consequently, this type of ADM does not effectively compensate for the insertion loss experienced by the optical signals in the add path of the ADM. Similarly, a lumped amplifier placed at the output side of the second circulator cannot effectively compensate for the insertion loss in the drop path because the optical signals reflected by the fiber gratings and dropped via the first circulator do not pass through the lumped amplifier at the output side of the second circulator. In sum, the lumped amplifier approach does not provide an efficient amplification scheme for ADMs. Furthermore, adding more lumped amplifiers to the various paths within an ADM only adds to the cost and complexity of the system.
These disadvantages are compounded if additional channels are added or dropped as part of a future service upgrade. More specifically, add/drop devices in typical lightwave communications systems are designed to accommodate a predetermined number of channels for adding and dropping, because the losses associated with adding and dropping must be accounted for in each of the output paths of the add/drop device. In particular, more loss is introduced as more channels are added and dropped. In order to provide a less lossy drop or add operation, some prior art ADMs utilize wavelength multiplexers and demultiplexers to further combine or split the optical signals in the add and drop paths, respectively. For example, the wavelength demultiplexer receives a composite optical signal and then demultiplexes and filters out each of the individual channels accordingly. However, wavelength multiplexers and demultiplexers impose several limitations on the add/drop capability of a system. Aside from being costly, these devices have a finite number of ports so that a future service upgrade, such as the removal or insertion of additional channels, requires a complete replacement of the multiplexer or demultiplexer. This approach results in an interruption of existing add/drop service while the hardware is being replaced to accommodate the new service. Moreover, the operation of these devices is not cost effective, because the entire optical signal is multiplexed or demultiplexed regardless of whether each of the individual channels is being used.
Consequently, prior art systems are limited because the add/drop capability cannot be expanded without significant redesign or reengineering. In particular, the lumped amplifier must be redesigned or additional lumped amplifiers must be added in order to compensate for the additional losses associated with the expansion of add/drop service. Additionally, service interruptions occur when individual components within the drop and add paths must be replaced to accommodate additional channel adds/drops. In all cases, this redesign results in increased cost, added design complexity, and disruption of existing add/drop service. Accordingly, there is a need for a loss-less, highly wavelength-selective optical add/drop system that is expandable to accommodate service upgrades without disrupting existing service.